Many electrical devices that are to be operated at a mains power supply network, i.e. in the power range from 300 W (Watt) to several 10th of kW (kilowatt), are designed to receive a DC input voltage. The mains supply usually is a single or multiphase AC voltage grid which has therefore to be converted into a DC voltage for supplying such devices. The DC voltage required by an electrical device is usually provided by an AC/DC power supply unit (PSU). Power supply units are available for a wide range of applications, such as for example computers such as servers or personal computers, storage devices and network industry as well as for telecom infrastructure. But they are also available for inductive cooking/heating systems or automotive chargers, particularly on-board chargers (OBCs) used in electrically driven cars and other EV (electric vehicles) or HEV (hybrid electric vehicles). Such OBCs charge the high-voltage traction battery used in such vehicles for providing the electrical power to the motor directly from the public AC power grid.
PFC converters are the front-end stages of many of today's AC/DC PSUs which operate directly from the AC mains. The PFC stage regulates the AC side power factor close to one (power factor correction) and it also controls the DC-link voltage which is the input voltage for the subsequent stages independently of the actual power flow to a constant value. The DC-link voltage is usually provided across a capacitor at the converters output. Most of those PFC stages operate according to the boost principle, i.e. the DC-link voltage in normal operating mode has always to be larger than any instantaneous value of the AC input voltage, which can be assumed to be sinusoidal over time with an amplitude ÛAC and an angular frequency ω.
A standard boost PFC converter includes a full bridge diode rectifier for rectifying the AC input voltage, followed by a boost inductance and a boost diode that is connected to the first output terminal. A controllable switch such as an IGBT, a MOSFET, thyristor or the like is connected between the boost inductance and the second output terminal and the DC-link capacitor is connected between the two output terminals. As already mentioned, the DC-link voltage in normal operating mode has to be larger than the AC input voltage at every single point in time. But before startup of the boost PFC converter the DC-link capacitance is usually discharged which means the voltage across the DC-link capacitor is usually 0 V (Volts). This means that the DC-link capacitance has to be precharged before the converter can be operated in its normal boost mode. In order to avoid high inrush currents, the input voltage may not directly be connected to the output capacitor, because the inductance of the boost inductor is quite small and therefore is not sufficient to limit the charge current sufficiently.
A known solution to precharge the DC-link capacitance of such a boost PFC converter is to provide a resistor R in the charge current path such as to limit the current flowing into the DC-link capacitor. Once the DC-link capacitor is fully charged, the resistor R is bypassed by closing a switch that is connected in parallel to the resistor R. The switch can be implemented as an electromechanical relay or by different types of switchable power semiconductors like e.g. IGBT, MOSFET or thyristor. Different locations for the resistor R are possible, either at the AC side, for example between an input terminal and the rectifier, or at the DC side, for example between the second output terminal and the rectifier. In certain applications a surge diode is also provided bypassing the boost diode and the boost inductance.
In another known solution two additional diodes are provided, each of them being connected in conduction direction from a different one of the input terminals of the converter to the charge current limiting resistor which itself is connected to the output capacitor. Further, two diodes of the rectifier are replaced by thyristors such that by switching OFF the thyristors the input terminals are disconnected from the boost inductance. Accordingly, the input current flows through the additional diodes and the resistor to the output capacitor. In order to control the thyristors a control unit is provided which is connected to the output capacitor such as to supply the control unit with the electrical energy to provide the control signals for the thyristors. Accordingly, since the output capacitor is not charged at the startup of the converter, the control unit is not powered up and therefore the thyristors are switched OFF. As soon as the output capacitor is charged to a certain level, the control unit is powered up. Then it is able to switch the thyristors ON such as to bypass the additional diodes and the resistor thereby stopping the precharging of the output capacitor and starting the normal boost operation of the converter.
Document EP 1 186 093 B1 (Ascom Energy Systems AG) discloses another solution for precharging the output capacitor. One of the rectifier diodes is replaced by a thyristor 18 and a precharge path 2 including a diode 20 and a resistor 19 is provided in parallel to that thyristor 18. At the startup, the thyristor is switched OFF such that the current flows through the precharge path 2 to the output capacitor. A control circuit including a voltage divider 25, a reference voltage 26, a limiting diode 27 and a comparator 28 provides the control signals for controlling the thyristor 18.
A main drawback of the prior art is that a resistor is needed to limit the charge current during the charging of the output capacitor. During the precharge interval large currents flow through this resistor and therefore a lot of power has to be dissipated into heat by it. For thermal reasons this resistor has to have a rather bulky volume which also results in certain constraints with regard to the component layout of the power supply and the thermal connection of the resistor. Another drawback is the fact that, apart from the resistor itself, additional components are needed such as for example diodes, a controllable switch or even a rather complex control circuit, which not only results in increased space requirements but also increased manufacturing costs.